The subject matter of the present invention is a method to extract or recover hydrochloric acid from hydrochloric acid solutions containing metals by means of pyrohydrolytic treatment of the solution, followed by absorption and/or condensation of the gaseous hydrogen chloride thus formed in order to form hydrochloric acid. A device for implementing the process according to the invention is also the subject of the present invention.
Hydrochloric acid solutions containing metal are often encountered in the metal industry, for example, during pickling of carbon steel. These solutions contain free hydrochloric acids and metals dissolved as chlorides, such as iron. Hydrochloric acid solution containing approximately 18% HCl and as little iron as possible is typically added to the final pickle section in counter-current to the strip. The spent pickling solution containing approximately 120 g/I Fe from the first pickle section is removed and regenerated.
Several methods have already been developed by which the hydrochloric acid from the spent pickling solution can be recovered.
AT395312B describes a process where the acid is recovered by spray roasting of the solution containing metal, followed by absorption and/or condensation of the gases thus formed in an aqueous absorption solution. The metal oxides generated during pyrohydrolysis are removed at the base of the spray roaster.
EP 0775760 describes a similar process for recovery of acid by means of pyrohydrolytic treatment, where the waste pickling liquor undergoes pre-concentration by evaporation before pyrohydrolysis.
The set-up of these systems is known in specialist circles or from AT395312B and EP 0775760, thus it is not described in more detail here.
DE 19 32 765 discloses a method for regeneration of HCl from aqueous iron chloride solutions. Here, the iron chloride filtrate is divided into two partial flows. The first partial flow is roasted, and the second partial flow is fed to an absorber.
U.S. Pat. No. 3,399,964 A discloses a method to regenerate pickling liquor containing hydrochloric acid and iron. Here, a partial flow of the pickling acid is circulated in an absorption loop in order to increase the concentration of the pickling liquor, while the other partial flow is fed to a fluidized bed reactor after pre-concentration by evaporation.
The energy requirement for HCl regeneration in a conventional plant—depending on the size of the plant—is approximately 650-700 kcal per liter of waste pickling liquor fed to the plant. Normally, a gaseous fuel is used to fire the plant.
One disadvantage of the current HCl regeneration process by spray roasting is that the maximum possible metal concentration is not achieved in pre-concentration of the waste pickling liquor (Venturi loop), which means that an unnecessarily large amount of water is evaporated, causing a very high energy requirement for acid regeneration.